About Sample & Hold circuit

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I am trying to study a Sample&Hold Citcuit， as shown in Fig.1, Vosc- and Vosc+ are the signals (opposite signals) that control J3~J6 generated by the pluse generator respectively, and the in Fig.2, my understanding It is when J4 and J5 are closed at the same time, the voltage of point VM+ should be the input voltage V+, but I don't know why, when I simulated in LTspice, the voltage of VM+ is almost the same as the signal Vosc- (as shown on the left side of Fig.1), I don't know what caused this, is there any solution? Thank you so much.

Fig.1

Fig.2
 

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Switch phasing must ensure there is no overlap which
would ohmically mess up the transfers-of-charge.

4nF is pretty chubby. Maybe FET on resistance makes
a time constant that is lower than your sampling pulse
pulse width and so incapable of a clean single pulse
sample. Zoom in on a single clock cycle and assess
the sampling settling, and the hold droop.

But how you expect to control -15V signal with FETs
operating between Gnd and positive supply, I do not
see offhand. Maybe you should back up a few steps
and figure out the regions where those FETs have
actual, proper authority / operation - including that the
controlling sources are doing the right thing at those
various bias-points.
 

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